The invention relates to a process for eliminating mercury and possibly arsenic from a hydrocarbon-containing feed, comprising at least: a first step for vaporising the feed, followed by condensing the vaporised feed, then a catalytic step carried out in the presence of hydrogen that can possibly capture arsenic, and a step for adsorbing mercury on a mercury capture mass.
Liquid condensates (by-products from gas production) and certain crude oils are known to contain a variety of metallic trace compounds, usually in the form of organometallic complexes. Such metallic compounds are usually poisons for the catalysts used in processes for transforming such cuts into commercial products. Mercury is particularly poisonous as regards the activity of precious metals. It is also highly corrosive towards aluminium parts, and to seals and welds.
It is thus advantageous to purify feeds for sending to processes for transforming condensates or crudes to avoid entraining mercury and possibly arsenic. Purification of the feed upstream of treatment processes can protect the whole of the facility.
The applicant has previously proposed a process for eliminating mercury from hydrocarbons acting as feeds for a variety of treatment processes. U.S. Pat. No. 4,911,825 describes a process for capturing mercury and possibly arsenic using a two-step process. The first step consists of bringing the feed, in the presence of hydrogen, into contact with a catalyst comprising at least one metal selected from the group formed by nickel, cobalt, iron and palladium. Mercury is not, or is only slightly, captured by the catalyst but it is activated on that catalyst so as to be captured in a second step by a mass comprising sulphur or a metallic sulphide.
U.S. Pat. No. 5,384,040 describes a process for eliminating mercury from a liquid hydrocarbon feed, comprising two stepsxe2x80x94a step for transforming compounds containing mercury into elemental mercury, and a step for fractionating the effluent from the first step. The metallic mercury from the first step is distributed in at least two cuts: at least one light fraction that is enriched in mercury and has a boiling point of less than 180xc2x0 C., which is treated using a metallic mercury adsorption mass, and at least one heavy fraction with a boiling point of more than 180xc2x0 C., with a reduced mercury content.
Japanese patent JP-07-103377 describes a process for eliminating mercury contained in liquid hydrocarbons comprising a first feed heat treatment step carried out at a temperature of 200xc2x0 C. or more to decompose all of the mercury species present in the feed to mercury metal, then a second step consisting of bringing the heated liquid hydrocarbon into contact with an adsorbent including a molybdenum sulphide at a temperature not exceeding 200xc2x0 C.
U.S. Pat. No. 4,094,777 describes a process for capturing mercury in its metal form, in the gas or liquid phase using an adsorbent mass comprising a copper sulphide and possibly a silver sulphide disposed in a fixed bed.
U.S. Pat. No. 5,989,506 describes a process for removing mercury from a feed. This process comprises fractionation of the feed into a gas fraction comprising C1-C3 hydrocarbons and water and a liquid fraction comprising C3+ hydrocarbons and water, then a separate treatment of the two fractions using regeneratable adsorbents in a sequential manner.
The invention concerns a process for capturing mercury and possibly arsenic comprising at least:
a) vaporising (or flashing, step a1) then condensing the hydrocarbon-containing feed (step a2) without separating said feed;
b) treating the effluent from step a2), comprising at least one step for bringing said effluent into contact with hydrogen and a catalyst;
c) a step consisting of passing the effluent from step b) over a mercury capture mass.
By vaporising the feed then condensing it, sludge is separated from said feed. Further, this step can practically completely or completely eliminate mercury from this sludge. The vaporised effluent is then condensed into a single cut, free of sludge, but slightly enriched with the mercury from the sludge.
The process according to the invention thus comprises a step for vaporising the feed to be treated by heating (step a1) to temperatures that are preferably close to the end point of the feed in question, i.e., close to the temperature beyond which all of the feed has been vaporised with the exception of a residue that is general pasty and essentially constituted by sludge. These temperatures are generally in the range 20xc2x0 C. to 600xc2x0 C.
One of the aims of the process of the invention is to eliminate the sludge present in the feed and to avoid separate treatments of the several fractions resulting from the feed. An increase in the concentration of mercury has been observed in the cut obtained after evaporation by heating. This increase in the mercury content is obtained by decomposition of the organometallic mercury compounds and/or thermal decomposition of sludge containing mercury.
After condensing the vaporised feed, the condensate obtained is sent to a catalytic treatment step (step b) which can activate the mercury compounds and can also possibly capture arsenic, preferably eliminating at least 90% by weight of the arsenic contained in the condensate, more preferably at least 95% by weight, still more preferably at least 98% by weight, and highly preferably at least 99% by weight. Step b) is followed by mercury capture on an adsorbent mass (step c)), which preferably eliminates at least 90% by weight of the mercury contained in the condensate, more preferably at least 95% by weight, still more preferably at least 98% by weight and highly preferably at least 99% by weight.
Vaporisation/condensation steps a1) and a2) advantageously concentrate in the heaviest fraction particles in suspension which constitute the sludge and are formed from solid mineral compounds (for example silica) and/or heavy hydrocarbons in the condensed form. Further, the mercury previously present in the metallic or organometallic form in this sludge is thermally decomposed during vaporisation.
The invention thus concerns a process for capturing mercury and possibly arsenic comprises at least:
a) vaporising (or flashing, step a1)) said hydrocarbon feed followed by condensing. This vaporisation is carried out in a temperature range generally in the range about 20xc2x0 C. to 600xc2x0 C. and at a pressure in the range 0.1 to 5 MPa, more preferably in the range 0.1 to 2 MPa. The temperature is selected as a function of the nature of the properties of said feed, i.e., as a function of the end point of the feed. In general, the temperature selected is slightly lower or slightly higher than the end point. Preferably, the temperature is in the range from the temperature of the end point of the feed reduced by 20xc2x0 C. to the temperature of the end point of the feed increased by 20xc2x0 C., more preferably in the range from the end point reduced by 10xc2x0 C. to the end point increased by 10xc2x0 C. The effluent vaporised during step a1) is then condensed (step a2) at a temperature lower than that of step a1) and advantageously in the range xe2x88x9210xc2x0 C. to 500xc2x0 C. and at a pressure in the range 0.1 to 5 MPa, more preferably in the range 0.1 to 2 MPa.
b) A step (step b)) comprising bringing the heavy cut into contact with hydrogen in the presence of a catalyst. This step transforms mercury organometallics, in other words it activates the mercury and can also optionally capture arsenic. Advantageously, for example, the Applicant""s process described in U.S. Pat. No. 4,911,825 can be used, which consists of bringing the feed into contact with hydrogen in the presence of a catalyst comprising at least one metal selected from the group formed by nickel, cobalt, iron and palladium. Preferably, at least 50% of said metal is in the reduced state, i.e., in the metallic state, but it can also optionally be in the sulphide form. The metal is preferably supported. More preferably, the catalyst also comprises a support selected from the group formed by: alumina, silica, silica-aluminas, zeolites, activated charcoal, clays and aluminous cement. Mercury is not (or is only slightly) captured by the catalyst but it is activated on the catalyst so that it can be captured in the second step described below. When arsenic is also to be captured, the catalyst is more preferably nickel-based, preferably in the sulphide form and deposited on a support. The metal content of the catalyst is preferably in the range 0.1% to 60% by weight, more preferably in the range 5% to 60% by weight, and more preferably in the range 5% to 30% by weight. When palladium is present, it is preferably present in the range 0.01% to 10% by weight, more preferably in the range 0.05% to 5% by weight. This step is preferably carried out at a temperature in the range 130xc2x0 C. to 250xc2x0 C., more preferably in the range 130xc2x0 C. to 220xc2x0 C., still more preferably in the range 130xc2x0 C. to 180xc2x0 C. The operating pressure is generally in the range 0.1 to 5 MPa, preferably in the range 0.2 to 4 MPa, more preferably in the range 0.5 to 3.5 MPa. The hydrogen flow rate is generally in the range 1 to 500 hxe2x88x921 (volume per volume of catalyst per hour, under normal temperature and pressure conditions).
c) A step c) consisting in passing at least a portion of the effluent from step b) over a mercury capture mass comprising, for example, sulphur and/or at least one sulphur-containing compound, i.e., passing said effluent over at least one adsorbent based, for example, on a metallic sulphide deposited on a support. Advantageously, the technique described in U.S. Pat. No. 4,094,777 or U.S. Pat. No. 4,911,825 is used, preferably a capture mass containing sulphur and possibly a metal that is at least partially in the form of a sulphide. This metal is preferably selected from the group formed by: copper, iron and silver. The quantity of metal that is combined or otherwise in the sulphide form is preferably in the range 0.1% by weight to 20% by weight with respect to the total weight of the capture mass. The amount of elemental sulphur, combined or otherwise, of said mass is advantageously in the range 1% by weight to 40% by weight, and preferably in the range 1% by weight to 20% by weight with respect to the total weight of said mass. Said mass can also comprise a support preferably selected from the group formed by: silica, alumina, silica-aluminas, zeolites, clays, activated charcoal, and aluminous cements. This step is generally operated at a temperature in the range 0xc2x0 C. to 175xc2x0 C., preferably in the range 20xc2x0 C. to 120xc2x0 C., more preferably in the range 20xc2x0 C. to 90xc2x0 C. The operating pressure is generally in the range 0.1 to 5 MPa, preferably in the range 0.2 to 4 MPa, and more preferably in the range 0.5 to 3.5 MPa. The space velocity with respect to the capture mass is generally in the range 1 o 50 hxe2x88x921 (volume of effluent from step b) per volume of capture mass per hour), more preferably in the range 2 to 40 hxe2x88x921, and still more preferably in the range 1 to 30 hxe2x88x921.